THE ORDOVICIAN EXTINCTION 444 million years ago
TOTAL GENERA THROUGH THE PHANEROZOIC ERA The Cambrian to the Present PERMIAN TRIASSIC CRETACEOUS Holocene The Present Miocene DEVONIAN ORDOVICIAN Mississippian Jurassic Silurian Cambrian PANGAEA Super Continent
Georgia LAURENTIA Proto-North America The Cambrian World 500 million years ago
Georgia LAURENTIA Proto North America The Ordovician World 444 million years ago
ORDOVICIAN LIFE Ordovician strata are characterized by numerous and diverse trilobites and conodonts (phosphatic fossils with a tooth-like appearance) found in sequences of shale, limestone, dolostone, and sandstone. In addition, blastoids, bryozoans, corals, crinoids, as well as many kinds of brachiopods, snails, clams, and cephalopods appeared for the first time in the geologic record in tropical Ordovician environments. Remains of ostraderms (jawless, armored fish) from Ordovician rocks comprise some of the oldest vertebrate fossils. Despite the appearance of coral fossils during this time, reef ecosystems continued to be dominated by algae and sponges, and in some cases by bryozoans. However, there apparently were also periods of complete global reef collapse due to global disturbances. ORDOVICIAN EXTINCTIONS The Extinction at the end of the Ordovician eliminated an estimated 85 percent of all Ordovician species. This extinction interval ranks second in severity to the 95% extinction at the end of the Permian! The end-ordovician extinction is generally attributed to two factors: the first wave of extinction may be related to rapid cooling at the end of the Ordovician Period, and the second phase is widely regarded as having been caused by the sea-level fall associated with the glaciation. The drop in sea level would have drained the large epicontinental seas and reduced the available habitat for organisms that favoured those settings. No concentration of iridium has been identified near the extinction that would suggest a bolide (meteorite or comet) impact like the one identified at the end of the Cretaceous Period.
The Ordovician mass extinction has been theorized by paleontologists to be the result of a single event; the glaciation of the continent Gondwana at the end of the period. Evidence for this glaciation event is provided by glacial deposits discovered by geologists in the Saharan Desert. By integrating rock magnetism evidence and the glacial deposit data, paleontologists have proposed a cause for this glaciation. When Gondwana passed over the north pole in the Ordovician, global climatic cooling occured to such a degree that there was global large-scale continental resulting in widespread glaciation. This glaciation event also caused a lowering of sea level worldwide as large amounts of water became tied up in ice sheets. A combination of this lowering of sealevel, reducing ecospace on continental shelves, in conjunction with the cooling caused by the glaciation itself are likely driving agents for the Ordovician mass
The end Ordovician (Hirnantian) extinction was the first of the five big Phanerozoic extinction events, and the first that involved metazoan-based communities. It comprised two discrete pulses, both linked in different ways to an intense but short-lived glaciation at the South Pole. The first, occurring at, or just below, the Normalograptus extraordinarius graptolite Biozone, mainly affected nektonic and planktonic species together with those living on the shallow shelf and in deeper water whereas the second, within the N. persculptus graptolite Biozone, was less focused, eradicating faunas across a range of water depths. In all about 85% of marine species were removed. Proposed kill mechanisms for the first phase have included glaciallyinduced cooling, falling sea level and chemical recycling in the oceans, but a general consensus is lacking. The second phase is more clearly linked to near-global anoxia associated with a marked transgression during the Late Hirnantian. Most recently, however, new drivers for the extinctions have been proposed, including widespread euxinia together with habitat destruction caused by plate tectonic movements, suggesting that the end Ordovician mass extinctions were a product of the coincidence of a number of contributing factors. Moreover, when the deteriorating climate intensified, causing widespread glaciation, a tipping point was reached resulting in catastrophe.
ORDOVICIAN LIMESTONE (I-59 Road Cut NW Georgia)
ORDOVICIAN LIMESTONE (I-59 Road Cut NW Georgia)